For outdoor luminaires, it is important to solve the problem of glare, because glare causes many road safety problems. Currently, the glare effect of outdoor luminaires such as road luminaires is controlled by limiting the luminous intensity in certain directions. For example, the luminous intensity is limited to remain below a certain level at large emission angles, e.g. emission angles larger than 80 degrees. The emission angle is defined as a vertical angle above the direction pointing directly downward from the outdoor luminaire. Hereinafter, a vertical angle above the direction pointing directly downward from the luminaire is also referred to as vertical angle for short.
With the continuous increase of the luminous efficiency of high power LEDs in recent years, more and more outdoor LED luminaires have been set up for road or urban lighting.
A typical LED luminaire for road lighting is depicted in FIG. 1. The appearance of the LED luminaire 100 differs substantially from a traditional luminaire for road lighting such as the popular HID luminaire. The traditional luminaire generally has a curved cover plate, because, firstly, its light source is large in size and the curved cover plate can provide sufficient space for disposing the light source, and secondly, its light source generates a large amount of heat and the cover plate should be kept at a certain distance from the light source to avoid overheating. Contrary thereto, LED light source 110 is small in size, so that less space is required, and the heat generated by the light source is mostly dissipated from the back of the light source. Hence, a curved cover plate is unnecessary for the LED luminaire. Accordingly, a flat cover plate 120 is used for the LED luminaire because the flat cover plate is easier to manufacture and costs less than the curved cover plate. Moreover, the flat cover plate 120 is also advantageous in limiting luminous intensity at large emission angles.
As mentioned above, the LED luminaire has a higher luminous efficiency than the traditional luminaire. However, as regards outdoor LED luminaires, such as LED luminaires for road or urban lighting, there are more complaints from end users, including vehicle drivers and pedestrians, that they cause glare.